In many applications of reversible electric motors for moving parts that may accidentally be blocked by various unpredictable causes, it may be necessary to detect the presence of an obstacle that stops the motion to protect either the motor and/or the transmission system from overloads that may damage them, or to prevent injuries when the obstacle is a part of the human body (for example a finger, a neck etc.). Examples include the detection of an accidental block of an electric motor dedicated to lift and lower a glass window of a car, a garage door and the like.
FIG. 1 schematically depicts a motorized car window system. The glass pane 2 of a door 1 of a car is moved by a DC motor 3 (M). The mechanical transmission 4 between the motor and the glass is sketched with a dashed line. The motor 3 is driven by a full-bridge stage 5, including four switches SW1, SW2, SW3 and SW4 that may be driven in pairs depending on the desired direction of rotation of the motor, and thus of the desired direction (up or down) of motion of the glass pane 2. The switches SW1 . . . SW4 are connected in series along the two parallel branches of the bridge, between the supply nodes 6 and 7. The switches SW1, SW3 are connected in series to form a first branch between the nodes 6 and 7, and the switches SW1 and SW4 are connected in series to form a second branch between the nodes 6 and 7. The motor 3 is connected to the intermediate nodes 8 and 9 of the two branches of the bridge. Depending from the configuration of the four switches, the current IM forced through the motor will circulate either in a direction or in the opposite direction determining the direction of rotation of the motor.
The switches SW1 . . . SW4 are driven by a control circuit 10 CTRL as a function of a start command (H/L) including the information of the desired direction of rotation. The control circuit 10 produces the four control phases for the respective switches SW1 . . . SW4. The bridge 5 and the circuit 10 are generally powered by a regulator 11 (REG) of the car battery (not depicted) voltage (Vbat) providing a regulated DC voltage Vcc. If required, the supply voltages of the bridge 5 and of the control circuit 10 may even be different.
Generally, the current IM flowing in the motor 3 is monitored and the instantaneous current information is input to the control circuit 10. In FIG. 1, the sensing of the current IM is indicated by an ampere meter symbol 12 in series with the motor 3. In practice, this sensing may be done on a sense resistor connected in series to one or to both branches of the bridge 5.
The instantaneous current information is used by the control circuit 10 to protect the motor against accidental overloads that may damage it. The current sensing may be substituted or associated to the sensing of the temperature of the motor to switch off electric supply when the temperature exceeds a certain limit. In any case, current and/or temperature sensing involve the fixing of thresholds and are generally unsuitable for sensing torque variations of the motor.
For instance, in a motorized window or door with automatic detection of the end of the run it may also be required to have a so-called “anti-pinch” function to prevent crushing with excessive force an accidental obstacle that could cause injury to a limb or other part of a human body. Such an anti-pinch function must also reverse the motion to open the window or door upon detecting a pinch to free the obstacle as quickly as possible.
A problem is that comparing the instantaneous current or speed with a threshold is not sufficient to create a reliable detection of an excessive torque. In effect, the motor current IM may vary for other causes than that of an obstacle blocking the movement of the part moved by the motor. For a DC motor it is possible to write the following relations:VM=LM·dIM/dt+RM·IM+EM, where VM is the supply voltage of the motor, IM is the current, LM is the inductance of the motor, RM is the resistance of the motor and EM is the back electromotive force;EM=k·SM, where SM is the rotation speed of the motor and k is a constant;KM=k′·IM, where KM is the motor torque and k′ is a constant; andKM−KR=JM·dΩM/dt, where KR is the resisting torque, ΩM is the speed of the motor and JM is the moment of inertia of the rotor.
The back electromotive force of the motor is thus directly proportional to the rotation speed thereof while the motor torque is directly proportional to the current. Except where expressly indicated, the word torque in the following description indicates the motor torque against the resisting torque representing the mechanical load (e.g. the weight of the glass window) plus the friction (of the edges of the glass pane sliding in the door guides).
FIG. 2 is a diagram showing the range of variation and the general characteristic of the current IM flowing in the motor when closing a car window by lifting it up to its end of run. For sake of simplicity, oscillations of the current due to noise (switchings of the motor brushes on the collector) are not shown. The curve 21 (solid line) illustrates a mean functioning, that is the closing of the glass pane under mean hygrometric and temperature conditions.
The user pushes a command button producing a close-window command that is sent to the controller 10. The switches SW1 and SW4 or SW2 and SW3, depending on the transmission, are turned on and the motor is powered. When the motor is switched on (instant t0) the current IM flowing in its windings increases though the speed is still null. The torque that is proportional to the current IM increases. At a time t1, the current IM drops because the motor is starting to rotate. The start of rotation reduces the torque and thus the absorbed current, until the time t2 when the current raises again. The instant t2 coincides with the moment at which the glass pane (initially wholly retracted inside the door) is braked by the horizontal gasket. Thereafter the torque and the current IM continue to increase but at slower rate during the lifting of the glass pane.
Upon approaching the end of the lift run current and torque drop because of the higher speed acquired by the glass pane and upon abutting against the upper glass edge receiving channel, at t4, the speed becomes zero and the torque increases abruptly. End of run detection is generally carried out via a position sensor that switches off the power to the motor with consequent drop to zero of the current. The curve 22 traced with short dashes represents the current (and thus the motor torque) in case of wet glass subjected to a reduced braking action by the gasket. The curve 23 traced with long dashes represents the case of iced glass. In this case, the torque necessary to lift the glass is definitely greater.
If a relatively yielding obstacle (for example, a wrist) interferes with the lift run of the glass pane, the run will be braked by the obstacle before being definitely stopped. The motor will decelerate more or less abruptly, which implies a drop of its back electromotive force and, as a consequence, a rise of the current for increasing the torque. In the absence of safeguard, an automatic lift of the glass could be dangerous. If a rigid (unyielding) obstacle is encountered, similar consequences are produced characterized by an instantaneous drop to zero of the rotation speed and thus a more abrupt increase of the torque.
A difficulty of reliably detecting a blocked condition or an abrupt deceleration is that for safety reasons the limit of tolerable compression of a limb or finger may correspond to or be even weaker than the required lifting force (accounting for the case of an iced glass window). Another difficulty is that the required lifting force may increase in time because of the aging of gaskets and linings and of progressive deformations of parts of the mechanism and of the window structure. Another difficulty is that abrupt variations of the motor torque stochastically occur when lifting the window pane, for example when the car is traveling a road with an irregular paving. In such an occurrence, illustrated by the dot and dashed peak 24 of the curve 21 of FIG. 2, the resisting torque increases abruptly when a wheel of the car rolls out of a pot hole. The torque and thus the current fall abruptly. When the wheel gets out of the hole, the inverse phenomenon is produced, that is the motor torque increases abruptly, and thus the current flowing in it.
For these reasons that render current monitoring approaches based on the fixing of thresholds hardly discriminating, other techniques have been proposed and are currently used. Typically, mechanical or electromechanical devices are used as anti-pinch sensors. Deformation of an elastic element between a mechanical detector and the moving glass, when a body is being pinched, is a known approach. This technique requires the use of an elastic conducting element of complex and costly installation.
Pinch detection may be performed by a switch fitted in a weather strip of the car window. The switch that is normally off switches on when the weather strip is subjected to a certain force. This approach is rather simple but the particular weather strip assembly implies a certain mounting complexity and maintenance may become necessary to maintain effectiveness. Moreover, in some cases this approach does not meet safety specifications. For instance, if the shape of the window determines a very slanted weather strip, a pinching force may not be applied in a direction sufficiently orthogonal to the strip, as shown in FIG. 3. Therefore, the pressure necessary to activate the switch may not be reached or may exceed the specified maximum value.
Alternative approaches are generally based on monitoring the motor speed by the use of speed sensors (Hall effect sensors, encoders, and alike). When a pinch condition occurs, the motor is blocked and its speed becomes zero. Therefore, a pinch condition causes an abrupt speed variation, that may be detected by speed sensors. Yet another known approach includes processing a number of operating parameter measurements such as temperature, supply voltage, motor speed and position of the motorized part for comparing the actual displacement of the moved part in respect to pre-established models. This approach requires a powerful calculator and a large number of sensors for measuring the parameters necessary for the calculations, besides the definition of particularly complex models in consideration of aging effects.